Orthopedic Implants - roslerblog

orthopedic implants

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Orthopedic ImplantsSurface Finishing Guidebook

surface finishing guidebook

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Fueled by more active lifestyles and increased life expectancy,

the market for knee, hip, and other replacement body joints

is on the rise. With more than $19 billion in annual worldwide

sales, implants for joint reconstruction make up nearly 40 per-

cent of all orthopedic product sales.

Thanks to significant advancements in materials and new or im-

proved surface finishing technologies, today’s artificial hips and

knees can last more than 20 years, giving the recipient decades

of comfort and agility.

Parts that are finished using modern mass finishing and shot

blasting methods are playing a key role in extending the life

span of orthopedic implants. Steps that include cleaning, de-

burring/edge radiusing, surface smoothing, post-casting sur-

face preparation, machining, CNC grinding, and of course final

finishing are making big differences in the quality and perfor-

mance of such products.

Executive Summary

In this guidebook, we will offer a primer on mass finishing and

shot blasting technologies, including a look at the equipment

and methods specifically used in the surface refinement of joint

reconstruction implants. To highlight the versatility of the two

surface finishing methods, we will illustrate in further detail how

mass finishing and shot blasting processes add value in the

production and processing of specific types of implants.

Please contact us for other guidebooks that cover

spinal implants, trauma implants, or medical instruments.

orthopedic implants

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Why quality matters for orthopedic implants p 4

Types of orthopedic implants commonly used in joint reconstruction p 4

Required quality and performance characteristics of orthopedic implants p 5

Materials used for manufacture of joint reconstruction implants p 6

The finishing quality of joint reconstruction implants must meet the most stringent standards p 7

The benefits of mass finishing and shot blasting for finishing the surface of orthopedic implants p 8

The smooth and perfectly polished results of the mass finishing process p 9

How shot blasting cleans and prepares surfaces and extends implant life spans p 11

Mass finishing and shot blasting results support improved quality and efficiency p 13

A look at the most common finishing applications for joint reconstruction implants in more detail p 14

Orthopedic Implants: A Look Ahead p 19

Contents


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It would come as no surprise to most casual observers that

quality standards for orthopedic implants are among the most

demanding to be found in any manufacturing operation. Due

to the need to facilitate millions of movements between mul-

tiple parts, joint reconstruction implant requirements are es-

pecially rigorous.

Aside from choosing the right materials and producing these

parts to exacting, minimal dimensional tolerances, surface fin-

ishes, too, must be honed to perfection. This often involves

two distinct finishes on the same component. For example, the

front surface section of an artificial knee femoral must be ex-

tremely smooth, as it has to move freely on the tibia plate; con-

versely, on the backside of these implants, a textured, some-

As the name already suggests, the purpose of orthopedic joint

reconstruction implants (also called “endoprosthetics”) is to re-

place damaged joints by inserting a manufactured replacement

into the body. The most common of these replacement joints

are artificial knees and hips, which constitute almonst 90 per-

cent of the worldwide demand of joint implants. But, as a result

of various technological breakthroughs, other extremity joint

implants for ankles, shoulders, elbows, hands, feet, and jaws,

have been growing at an accelerated rate.

Unlike trauma implants, which may be removed after a bone

has healed, the goal for joint reconstruction implants is to re-

main permanently in the body to restore normal, pain-free

function of the respective joint. The longevity of these implants

has been constantly improved, with a working life today of

more than 20 years.

Why Quality Matters for Orthopedic Implants

what rougher surface must be used to allow osseointegration

of the implants into the bone.

In fact, orthopedic implants may have to undergo multiple fin-

ishing operations during the manufacturing process. These

operations can range from simple cleaning, deburring/edge

radiusing and surface smoothing after casting, forging and ma-

chining; to surface texturing, preparation for special coatings

and, frequently, high-gloss polishing. Some implants are even

shot peened to induce a compressive residual stress that will

further extend their service life

Types of Orthopedic Implants Commonly Used in Joint Reconstruction

Joint reconstruction can be a particularly challenging process,

because the implants must allow free movement of the joint

and, at the same time, must be firmly attached to the respec-

tive bones.

orthopedic implants

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BiocompatibilityThe implants must be compatible with the living tissue by not being toxic, injuri-ous, or physiologically reactive and not causing immunological rejection. This includes corrosion resistance to prevent reaction with bodily fluids.

High tensile strength and long life spanJoint reconstruction implants are exposed to considerable static and dynamic loads. They must withstand these loads for a lifetime without ever breaking.

Low friction at the joint contact areasThis requires very smooth, polished surfaces of those joint areas that are moving against each other. e.g. between femoral and tibia or femoral head and lining of the acetabular cup.

No sharp edges. All edges must be rounded

Sharp edges can cause ruptured blood vessels and blood clots during insertion and the healing process.

High osseointegration at surface areas, which are implanted into the bone

This requires a somewhat textured, rougher surface to allow the bone tissue at-taching itself to the implant.

Low dimensional tolerancesThe implant components must be very precise so that the fit and function of the joint is not compromised.

Joint reconstruction implants are subject to the same zero-

defect performance and reliability standards as any other

implants. However, because two components are always in-

Required Quality and Performance Characteristics of Orthopedic Implants

teracting with each other, dimensional accuracy is of particu-

lar importance. Other characteristics of orthopedic implants

include:


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To date, the most common materials have been titanium,

titanium alloys and cobalt-chromium alloys. Both materials are

very tough, resistant to corrosion, highly biocompatible, and

absolutely reliable.

While stainless steel often is used for trauma implants, it is not

very common in joint reconstruction implants because of its

limited long-term ability to withstand corrosion in the human

body. Plastics, primarily polyethylene (for example, UHMWPE),

have also increased in favor for joint reconstruction implants.

Materials Used for Manufacture of Joint Reconstruction Implants

The interface between two joint sections can consist of metal-

on-plastic, metal-on-metal, ceramic-on-plastic or ceramic-on-

ceramic.

For example, the section of the knee tibia plate that interacts

with the femoral component is protected with a polyethylene

liner. Likewise, the acetabular cup of a hip implant can be

lined with polyethylene, whereas the femoral head on the

hip stem interacting with the acetabular cup may be made of

metal.

Let’s take a closer look at some alternative materials sometimes used in the manufacturing process:

Because of its excellent wear

characteristics and the best

biocompatibility among the known

implant materials, ceramic is

quickly becoming an excellent

alternative to titanium and

cobalt chrome.

Ceramic

Plasma coating is primarily used to

promote osseointegration on those

surface areas of implants that must

bond with the surrounding bone

tissue. To ensure perfect adhe-

sion of the coating, the respective

surface area must be textured

(“roughened”) by shot blasting.

Plasma Coating

Joint reconstruction implants

frequently receive a PVD coating,

mainly with titanium nitride (TiN).

Such coatings provide numerous

technical advantages, including:

� Improved wear resistance

� Reduced friction

� High biocompatibility

� Decorative colors

PVD (Physical Vapor Deposition)

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The Finishing Quality of Joint Reconstruction Implants must Meet the Most Stringent Standards

Additive manufacturing is rapidly evolving from a purely

prototyping method into a full-fledged manufacturing sys-

tem, and there’s little doubt this trend will continue. This

trend has also reached the orthopedic implant industry.

Hip implants, but also knee and other joint replacements

already are made through additive manufacturing.

No doubt, this trend will continue over the coming years.

But additive manufactured components still pose enor-

mous finishing challenges: Not only must the support

structures and sintered metal particles be successfully re-

moved, but also a significantly higher surface roughness

be overcome. The initial surface roughness of a 3D print-

ed component can be as high as Ra = 2,000 microinches

(= 50 µm), while the initial surface roughness of a cast or

Joint reconstruction implants are subject to rigorous finishing

standards. Though choosing the right implant material is of

utmost importance, the significance of optimum surface treat-

ment cannot be overstated. This relates not only to the right

surface finish, but also total compliance with the specified tight

dimensional tolerances.

The functionality of a joint implant in the body is determined

by the perfect match between the various implant compo-

nents. This depends to a large extent on the surface treatment

procedure(s). The most important finishing requirements for

joint reconstruction implants are:

� No sharp edges or burs, to prevent rupturing of blood ves-

sels and blood clots during implantation and to expedite

the healing process

� An extremely smooth, polished surface on the implant ar-

eas that are interacting with each other, with Ra readings of

less than 1.0 microinches (= 0.025 µm)

� A textured, somewhat rougher surface finish on those ar-

eas that are implanted into the bone, to promote osseo-

integration

� Absolutely clean surfaces, without any contamination to

prevent infections

� A slightly textured surface as preparation for surface

coatings

� Longevity, i.e. improved resistance against tensile and

bending stress, is achieved by shot peening.

� Maintaining critical component dimensions during the

various finishing operations

Spotlight: Special Challenges of Finishing 3D Printed Joint

Reconstruction Implants

forged part amounts “only” to Ra = 120 – 320 microinches

(= 3 to 8 µm). Since it promotes bone growth, the higher

surface roughness is an advantage for surface areas im-

planted into the bone, but presents a challenge for those

surface areas that must be extremely smooth.


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Shot blasting and mass finishing have become indispensable technologies for surface preparation and finishing of joint reconstruction

implants. Their applications range from surface cleaning, deburring, edge radiusing after forging, casting, 3D printing, machining, etc.;

to surface preparation for different kinds of coatings, increasing the longevity of an implant, and placing a final, extremely smooth high

gloss finish on the implants before insertion.

The Benefits of Mass Finishing and Shot Blasting for Finishing the Surface of Orthopedic Implants

Mass finishing is a grinding system, leveraging the

pressure between the media and work pieces,

combined with the constant “rubbing” of the media

against the work pieces. This generates a grinding

and polishing effect, leaving a smooth surface finish

that can be as low as Ra = 0.8 microinches (0.02 µm)

Shot blasting is an impact system in which small

metal or mineral pellets are thrown onto the surface

of a work piece at speeds of 200 – 800 feet/second.

The impact on the work piece surface produces

the desired cleaning, peening or texturing effect.

For medical applications, mainly air and wet blast

systems are used which generally make a surface

rougher. The smoothest finishes achieved with shot

blasting are about Ra = 16 - 32 microinches

(= 0.4 to 0.8 µm)

Mass finishing is a grinding system

Shot blasting is an impact system

� The creation of homogeneous, all-around “isotropic”

(multi directional) finishes as opposed to “anisotropic”

(mono directional) surface structures produced by ma-

chining, belt and wheel grinding, drawing and extrusion.

� Ability to handle all materials, from the toughest metals

like titanium and platinum to all kinds of polymers and,

even, ceramics.

� Consistent, absolutely repeatable finishing results.

� Elimination of quality fluctuations inherent in manual or

other mechanical finishing methods

� Ability to choose from a broad equipment spectrum, from

simple, low-cost standalone machines to fully automated

finishing systems

Other features and benefits of shot blasting and mass finishing include:

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Mass finishing is a highly versatile finishing technology that can be used for a wide variety of different surface treatment operations.

As a result, it is no surprise that mass finishing processes are utilized at practically every manufacturing stage for all kinds of orthope-

dic implants. For example, mass finishing is used for:

Surface cleaning

Final polishing of knee femorals, femoral

heads and the inside of acetabular cups to

Ra = 0.8 micro inches (= 0.02 µm) as the last

finishing stage before implantation

The Smooth and Perfectly Polished Results of the Mass Finishing Process

Deburring/edge radiusing

Deburring and surface smoothing of various

implants after belt or CNC grinding

Polishing

Descaling and edge radiusing of hip stems, knee

femorals and other implants after forging or

casting, e.g. lost wax or investment casting

Polished knee implants fxtured in a

drag finsihing machine before being

removed

Surface Smoothing

Surface grinding makes the surface smoother.

It is frequently used as a preparation for subse-

quent polishing or blasting processes.


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Spotlight: The most common mass finishing machines

Considering their critical function in the body, joint reconstruction implants must have a perfect surface finish free

of any blemishes. Even the smallest nick or dent on a component will render them unsuitable for implantation. That

is why during the various finishing operations, the implants must not tumble over each other but must be firmly at-

tached to work piece holders, so that they avoid any contact with each other during the entire finishing process.

Drag Finishers

� Used For: Finishing of knee, hip and other joint reconstruction

implants

� Key Benefits: Extremely smooth and polished surface finishes on

a wide array of implants, with zero defects, and at surprisingly low

costs.

� How it Works: The work pieces are individually attached to work

stations on a rotating carousel and are dragged through a stationary

work bowl filled with media

Automated drag finishers with robotic work piece handling

� Used In: Fully-automated, robotic-driven processes

� Key Benefits: Creates autonomous, fully automated manufacturing

cells, which can be easily integrated into the overall manufacturing

process.

� How it Works: Robots equipped with specially designed gripper

systems load and unload the work pieces to and from the work

stations, equipped with a pneumatically-activated quick connect

coupling system.

Rotary Vibrators (Type DL)

� Used For: Finishing of ankle and shoulder implants and acetabular

cups, or small volumes of knee femorals and tibia plates.

� Key Benefits: Efficient processing of low production volumes

� How it Works: Machine equipped with two vibratory motors

mounted on the outside of the processing bowl. The work pieces

are mounted to fixtures in the bottom of the bowl. This eliminates

the risk of nicking and greatly reduces the risk of media lodging in

the work pieces.

Other mass finishing machines

For less delicate work pieces like screws, trauma and spinal im-

plants, certain medical instruments, etc., standard rotary or tub

vibrators or high-energy centrifugal disk finishing machines can

be utilized.

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How Shot Blasting Cleans and Prepares Surfaces and Extends Implant Life SpanLike mass finishing, shot blasting is an exceptionally versatile

surface treatment technology. Its applications range from gen-

eral cleaning after casting and forging, to shot peening, and

even cosmetic blasting for placing a fine, matte finish on the

work pieces.

For shot blasting of orthopedic implants, primarily air blasting,

but also wet blasting systems are employed: The blast media is

accelerated by compressed air and thrown at the work pieces

through a blast nozzle. This allows an extremely precise blast

pattern suitable for use with metallic, mineral, and organic blast

media. The most common shot blasting tasks for joint recon-

struction implants are:

� Surface cleaning after casting and forging

� Surface texturing as preparation for coating

� Cosmetic blasting

� Preliminary surface smoothing of 3D printed implants

� Shot peening

Shot peening increases component life by bombard-

ing the work piece surface with a stream of special

blast media. Each pellet forms a dimple in the surface,

creating a so-called compressive stress. The result:

Extension of the fatigue life of a component along with

increased load bearing capabilities and higher wear

resistance. Typical joint implants that are shot peened

include hip stems, femorals, and tibia plates.

Wet blasting uses a slurry consisting of water and abrasive or

non-abrasive media. Before being thrown at the work pieces

the slurry is accelerated by a special pump and compressed

air. Wet blasting generates no dust and can be calibrated

from very gentle to aggressive blasting. It creates no metal

erosion or media impregnation in the work pieces, no heat

warping of thin parts. It has a cleaning effect, even without

chemicals, allowing for very fine, textured finishes. A typical

use would be for the leveling of machining lines on tibia plates

and to texture the bone contact areas of implants

Shot Peening

Wet Blasting


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Surface cleaning

Descaling after forging, casting or heat treat-

ment. The surface usually becomes rougher.

Cosmetic blasting/texturing

A very fine, matte, anti-glare finished is

placed on components, for example,

surgical instruments.

The finishing tasks for shot blasting

Spotlight: Pre-smoothing of 3D printed components by blasting

Shot blasting normally makes a surface rougher, but since 3D printed parts have a very rough initial surface,

shot blasting has a smoothing effect. It is used for pre-smoothing prior to mass finishing.

Roughness readings

Raw part: Rz = 1,700 micro in. (43 µm)

After shot blasting: Rz = 880 micro in. (22 µm)

After mass finishing: Rz = 120 micro in. (3 µm)

Peening

Inducing a compressive stress in a component

surface makes it more resistant against general

wear and corrosion stress cracking.

Surface preparation for coating

Surface texturing creates better

adhesion of the coating material.

0

500 | 12,5

1000 | 25

1500 | 37.5

2000 | 50

Surf

ace

read

ing

(Rz)

in

mic

ro in

ches

| µ

m

Surface conditions of 3D printed parts

Raw part After blasting After mass finishing

Satellite Table Machines

� Used For: Hip stems, tibia plates and other orthopedic implants

� Key Benefits: Prevention of contact between work pieces

� How it Works: These blast machines are equipped with a rotary ta-

ble containing 4 to 12 independently rotating satellite stations. The

work pieces are attached to the satellites, which then pass through

one or multiple blast stations equipped with several blast nozzles.

Spotlight: The most common shot blasting machines for orthopedic implants

(dry blasting, wet blasting or shot peenig)

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Swing Table Machines

� Used For: Finishing of orthopedic implants.

� Key Benefits: Efficiency, prevention of contact between work pieces

� How it Works: Blast machines are equipped with a round table with

two rotating satellite workstations, one in the blast zone and one in

the load/unload zone. This allows loading/unloading one set of work

pieces, while another set is shot blasted.

Robotic Blasting Systems

� Used For: Complex part geometries and large part variety

� Key Benefits: Repeatable and precise process

� How it Works: The robot holds either the part or the blast nozzles

and follows the pre-prgrammed blast path to blast the part

Mass finishing and shot blasting results support improved quality and efficiencyBoth mass finishing and shot blasting are not only very cost-

effective, but also highly adaptable to customer needs. The

equipment spectrum ranges from small manual or semi-auto-

matic machines for low production volumes, to fully automatic

systems for high volume production.

The customer decides on the budget, and what degree of auto-

mation he desires. Generally, the costs for work piece fixtures

are manageable, and, due to the high degree of mechanization

and automation, personnel costs comprise only a small per-

centage of the total process cost.

The big savings are, however, achieved by the stability of the

finishing processes. Once a process has been established, it

produces absolutely repeatable, high-quality finishing results

with zero scrap rates, day-in and day-out.

Most machines can be equipped with automated work

piece loading and unloading to achieve � High production volumes

� Improved efficiency and throughput

� Improved cost efficiency


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Titanium Knee Femorals

Finishing Task (1) Surface smoothing after CNC grinding,

(2) Producing a pre-polish surface,

(3) High gloss polishing.

Mass Finishing Solution 3-stage process (cut-down, smoothing, polishing) in compact drag finishers with

(1) Fast cutting ceramic media,

(2) Special pre-polish plastic media and

(3) Organic dry polishing media

Finishing equipment 3 x Rosler drag finisher R 4/700 SF. Working capacity per machine: 12 knee femorals

per batch

Capacity Processing times:

(1) Cut-down = 80 min.,

(2) Fine grinding = 60 min.,

(3) Dry polishing = 30 min.

Capacity = about 9 pieces/hour

A look at the most common finishing applications for joint reconstruction implants in more detail

orthopedic implants

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3D Printed Acetabular Cups

Finishing Task Removal of machining lines from the concave cup surface area; creating a mat finish.

Threaded holes must not be affected

Shot Blasting Solution Manual processing in a wet blast cabinet with ceramic beads

Finishing equipment Rosler wet blast cabinet RWB 48 with turntable.

Capacity Processing time = 5 min. per piece.

Capacity = about 12 pieces/hour

Finishing Task Leveling of machining lines on the backside of titanium tibia plates

Shot Blasting Solution Processing in wet blast cabinet with ceramic beads

Finishing equipment Rosler indexing satellite table wet blast cabinet RWB 48-4/6 with 4 rotating satellite

work stations

Capacity Process time per batch = 8 min.

Capacity = app. 30 pieces/hour

Titanium Tibia Plates


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Titanium/Stainless Steel Hip Stems

Finishing Task The surface must be textured (roughened) for subsequent plasma coating of the stem

section

Shot Blasting Solution Processing in a satellite table air blast machine. Use of aluminum oxide blast media

Finishing equipment Rosler satellite table air blast machine R 1400 S-6 with 6 rotating satellite workstations.

To prevent contamination of the work pieces with ferrous particles all machine areas

coming in contact with the blast media must be made from non-ferrous material.

Capacity About 50 pieces/hour

Finishing Task Cleaning of the work piece surface after lost wax casting. Removal of all residues from

the casting process to achieve a pure metal surface finish. Next manufacturing step:

Machining

Shot Blasting Solution Blast cleaning in a blast cabinet. Use of aluminum oxide blast media. At this early

manufacturing stage the work pieces can gently tumble over each other.

Finishing equipment Rosler airblast cabinet RSKI 1400 equipped with two rotary drums

Capacity Approximately 200 work pieces/hour

Cobalt Chrome/Titanium Knee and Hip Implants

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Titanium Acetabular Cups

Finishing Task Removal of machining lines, high gloss polishing to Rz = 4 micro inches (= 0.1µm). Main

focus is on the concave surface area of the cup.

Mass Finishing Solution Two stage smoothing & polishing process in a rotary vibrator without center dome:

(1) Surface grinding and smoothing with pre-polish plastic media,

(2) Polishing with dry polishing media

Finishing equipment 2 x Rosler special rotary vibrators R 150 DL-2 with 2 vibratory drive motors on the out-

side of the processing bowl

Capacity Processing time stage

(1) surface smoothing: About 10 hours

(2) polishing: About 15 hours

Approximately 24 - 48 pieces/day

Finishing Task Removal of belting lines (220 grit belt), surface smoothing.

Mass Finishing Solution Finishing in a compact drag finisher with fast cutting ceramic media

Finishing equipment Rosler compact drag finisher R 6/1000 SF with 6 rotary work stations (spindles)

Capacity Batch size = 30 pieces; Processing time: About 20 minutes.

Capacity: About 70 – 80 pieces/hour

Titanium Alloy Hip Stems


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Ceramic Knee Femorals

Finishing Task Surface smoothing and polishing of ceramic knee femorals

Mass Finishing Solution 2-stage finishing process:

(1) Surface grinding/smoothing with fine grinding ceramic media,

(2) High gloss polishing with porcelain polishing media

Finishing equipment Rosler tub vibrator R 600/1000 TS. Work pieces are mounted to special work piece

fixture that they cannot touch each other during the finishing process

Capacity Batch size = 24 pieces (24 mounted to the fixture) Processing times stage

(1) Grinding & smoothing: About 24 h Processing time stage

(2) Polishing: About 2 h

Capacity: approximately 24 pieces/day

orthopedic implants

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Orthopedic Implants: A Look Ahead

For more information on orthopedic implant finishing, mass finishing, or shot blasting, visit us at www.rosler.us.

If you would like to send in your parts for FREE process development, please contact us

at 269-441-3000 or [email protected].

Due to their precision, efficiency, and economy, mass finishing

and shot blasting are considered to be an indispensable part

of the finishing process for a wide variety of joint replacement

implants in different manufacturing stages. These flexible ma-

chines can handle everything from general cleaning, deburring,

surface smoothing after casting, forging, stamping, machining,

heat treatment, or surface preparation for polishing or coating,

all the way up to the placement of the final finish on all kinds of

implants and medical devices.

Orthopedic implant manufactures are at the cutting edge of

medical technology. New materials and manufacturing tech-

niques and technologies are constantly evaluated to improve

the performance and longevity of the implants and reduce the

manufacturing cost. Two examples are the increased use of

ceramics as base material or coating, and additive manufactur-

ing. These new materials are generally a lot tougher and harder

than previously used materials, which presents new challenges

for finishing equipment. Suppliers of mass finishing and shot

blasting equipment and consumables have met these chal-

lenges head-on by modifying existing equipment designs or

developing brand-new machinery.

However, the biggest progress has been made in the field of

mass finishing media. There is now media on the market that

allows for the placement a high-gloss finish on components

made from cobalt-chrome, or titanium, after CNC grinding.

And these media even allow mirror polishing of ceramic work

pieces. Likewise, 3D printed components can be finished from

initial surface readings of >2,000 micro inches (50 μm) down to

less than 10 (0.25 μm).

In many cases the optimum manufacturing process includes

multiple production steps. Working with experts from the de-

vice manufacturer and the finishing system supplier will ensure

that the entire production process is considered when finding

the optimum solution. It might be possible to reduce the cycle

time in a previous higher cost production step to lower cost fin-

ishing step without jeopardizing the final product. Manufactur-

ers who understand the rigorous requirements for orthopedic

implants are well suited to help you achieving your goal of a

better finishing process.


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Rosler Metal Finishing USA, LLC1551 Denso Road

Battle Creek, MI 49037

Tel: 269-441-3000 Fax: 269-441-3001

[email protected]

tel:+12694413000

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